1. Field of the Invention
The present invention relates in general to an air/fuel ratio control system for an internal combustion engine and more particularly to a system for performing feedback control of an air/fuel ratio of a mixture to be supplied to the engine through detection of an air/fuel ratio of combusted mixture in two places, i.e., upstream and downstream of a catalytic converter to cope with a malfunction of an air/fuel ratio sensor upstream of the catalytic converter.
2. Description of the Prior Art
An example of a prior art air/fuel ratio control system for an internal combustion engine is disclosed in Japanese patent provisional publication No. 60-240840.
In the prior art air/fuel ratio control system, an engine intake air quantity Q and engine speed N are detected to compute a basic fuel supply quantity T.sub.p (T.sub.p =K.Q/N where K is constant). The basic fuel supply quantity T.sub.P is subjected to correction in accordance with engine temperature, etc. and to feedback correction by using air/fuel ratio feedback correction coefficient (air/fuel ratio correction quantity) which is set in response to a signal from an air/fuel ratio sensor (oxygen sensor) for detecting an air/fuel ratio through detection of an oxygen content in exhaust gases. The basic fuel supply quantity T.sub.P is further subjected to correction by battery voltage, etc. to finally determine a fuel supply quantity T.sub.I.
By outputting a pulse signal having a pulse width corresponding to the fuel supply quantity T.sub.I, to a fuel injection valve at a predetermined timing, a predetermined quantity of fuel is supplied to the engine.
The feedback correction of the air/fuel ratio in response to the signal from the air/fuel ratio sensor is performed so that the air/fuel ratio becomes closer to a target air/fuel ratio (stoichiometric air/fuel ratio). This is because the conversion efficiency (purification efficiency) of the catalytic converter which is disposed in the exhaust system for oxidizing CO, HC (hydrocarbon) and reducing NO.sub.X contained in the exhaust gases for purification of same, is set so that the catalytic converter can operate most efficiently under an exhaust gas condition resulting when a mixture of the stoichiometric air/fuel ratio is combusted.
The electromotive force (output voltage) produced by the above described air/fuel ratio sensor has such a characteristic that it changes suddenly adjacent the stoichiometric air/fuel ratio. Thus, by the comparison of the output voltage V.sub.O and the reference voltage (slice level) SL, it is determined whether the air/fuel ratio of the mixture is rich or lean with respect to the stoichiometric air/fuel ratio (i.e., richer or leaner than the stoichiometric air/fuel ratio). In case the air/fuel ratio is, for example, lean (or rich), the feedback correction coefficient .alpha. to multiply the above described basic fuel supply quantity T.sub.P is increased (or decreased) by increasing (or decreasing) a large proportional constant P at the first time of change of the air/fuel ratio to lean (or rich) and then increasing (or decreasing) a predetermined integration constant I gradually for thereby performing correction of increasing (or decreasing) the fuel supply quantity T.sub.I and controlling so that the air/fuel ratio becomes closer to the stoichiometric air/fuel ratio.
In the above described ordinary air/fuel ratio feedback control system, one air/fuel ratio sensor is installed on an exhaust manifold at a location as close as possible to a collective manifold portion where manifold branches are collected, in order to make higher the responsiveness of the air/fuel ratio sensor. However, the temperature at the collective portion is so high that the characteristic of the air/fuel ratio sensor is liable to vary. Further, mixing of the exhaust gases emitted from the respective cylinders is insufficient, so it is difficult to detect the average air/fuel ratio of the combusted mixtures emitted from the respective cylinders. Therefore, the accuracy in detection of the air/fuel ratio is not sufficiently high, thus deteriorating the accuracy in air/fuel ratio control.
In view of the above problem, it has been proposed to arrange another air/fuel ratio sensor at a location downstream of the catalytic converter and perform feedback control of the air/fuel ratio in accordance with the detection values by two air/fuel ratio sensors as disclosed in Japanese patent provisional publication No. 58-48756.
The air/fuel ratio sensor on the downstream side is far distant from the combustion chamber so that its responsiveness is not sufficiently high. However, the air/fuel ratio sensor on the downstream side is less affected by the balance of the exhaust gas components (CO, HC, NOx, CO.sub.2, etc.) and less exposed to the toxic components of the exhaust gases so that its characteristic is less variable. Further, for the reason of good mixing of the exhaust gases, the air/fuel ratio sensor on the downstream side enables detection of the average air/fuel ratio for all of the cylinders, i.e., more accurate and more stable detection as compared with that by the air/fuel ratio sensor on the upstream side.
Thus, a highly accurate air/fuel ratio control is performed by the combination of two air/fuel ratio feedback correction coefficients which are respectively set by computation similar to that described as above, based on detection values by two air/fuel ratio sensors or by compensating variations of the output characteristic of thee air/fuel ratio sensor on the upstream side by correcting the control constant (proportional part or integration part) of the air/fuel ratio feedback correction coefficient which is set by the air/fuel ratio sensor on the upstream side and the comparison voltage and the delay time of the output voltage of the air/fuel ratio sensor on the upstream side.
However, the air/fuel ratio control system using such two air/fuel ratio sensors encounters the following problems.
The conversion efficiency of the catalytic converter varies depending upon variations of the temperature. Particularly, variation of the conversion efficiency with respect to HC is large. This is because the oxygen storage ability of the catalytic converter varies largely in response to variations of the temperature. So, even if the air/fuel ratio of the upstream side exhaust gases flowing into the catalytic converter is unchanged, the oxygen storage ability of the catalytic converter at low temperature is insufficient to cause a lack of oxygen (O.sub.2) for reaction with hydrocarbon (HC) and a lowered HC conversion efficiency, whereas at high temperature the oxygen storage ability becomes higher to enable to attain a high HC conversion efficiency, i.e., the conversion efficiency varies depending upon variations of the temperature.
In this instance, when the correction quantity according to the air/fuel ratio of the downstream side exhaust gases is set larger, the air/fuel ratio is detected to be rather richer since the HC content in the exhaust gases at low temperature is large, resulting in that the lean correction quantity for correction of the air/fuel ratio toward lean becomes larger and at high temperature the lean correction quantity becomes comparatively smaller. Accordingly, in case a learning control of the correction quantity is made, the lean correction quantity is increased at high temperature, thus increasing the amount of NO.sub.x emission. When it is tried to make a learning control at every temperatures, the catalytic converter is disposed adjacent the road surface so there occurs such a case that the catalytic converter is suddenly cooled due to a splash of water, etc. Thus, it is difficult to estimate the temperature by means of a logic or temperature sensor and a good learning cannot be expected.
Further, during constant running of the engine (during constant speed running of the vehicle), the components of the exhaust gases on the upstream side of the catalytic converter are nearly constant and the reaction of the catalytic converter is maintained in a stable condition, so the air/fuel ratio of the exhaust gases on the downstream side of the catalytic converter is maintained constantly adjacent the stoichiometric air/fuel ratio, However, due to the influence of small variations of the air/fuel ratio caused, though momentarily, by the air/fuel ratio feedback control based on the detection value of the air/fuel ratio sensor on the upstream side or the influence of lodgment and dislodgment of oxygen (O.sub.2) stored in the catalytic converter by the oxygen storage effect, the output of the air/fuel ratio sensor on the downstream side causes a small hunting. Due to this hunting, each time when the output value exceeds the slice level, the correction of the proportional part, etc. based on the detection value by the air/fuel ratio sensor on the downstream side is switched toward increase or decrease of the air/fuel ratio, so there occurs such a case that the air/fuel ratio varies in timed relation to the above described hunting.
In this instance, if the correction quantity based on the detection value by the air/fuel ration sensor on the downstream side is sufficiently small, a variation of the air/fuel ratio can be small, but when the correction quantity is set large a variation of the air/fuel ratio becomes large.
In view of the above, when the correction quantity of the air/fuel ratio based on the detection value by the air/fuel ratio sensor on the downstream side is set small, a delay in the correction of the air/fuel ratio may be caused in case of occurrence of a sudden variation of the air/fuel ratio. Further, when a variation of the air/fuel ratio occurs in the reverse direction with respect to the correction quantity of the air/fuel ratio having been set before occurrence of the variation of the air/fuel ratio, the conversion efficiency of the catalytic converter becomes worse. Enumerated as an example of such a case are a case just after fuel cut, malfunction of the upstream side air/fuel ratio sensor (decrease of the rich side output voltage, etc.), malfunction of parts of a fuel line (fuel injection valve, airflow meter, etc.).